When creating a computer-animated motion picture, the animation team uses computers to create, view, and manipulate images used in the motion picture. The animation team makes judgments concerning the desired appearance of the images, and manipulates the images, based on how the images appear when displayed on the computer monitors.
The motion picture ultimately will be displayed to a theater audience using conventional motion picture film and projectors. Accordingly, the image data stored on computer must be converted into film images for projection in a theater.
Digital film recorders are used to convert the original, computer-generated images created by the animation team into images on photosensitive motion picture film. Conventional digital film recorders use a light source (such as a laser) to expose each frame of the film as necessary to produce the desired image in the frame. The film is then advanced to the next frame and the process is repeated. When a strip of film has been recorded, it is sent to a laboratory for development into a color negative and, ultimately, a positive print.
To create high quality computer-generated films, film producers must make certain that the ultimate appearance of the motion picture that is projected and viewed by an audience matches the appearance desired by the creative team. The color of each location on the exposed film should match, as closely as possible, the color of the corresponding picture element (“pixel”) on the display device used by the artist who created the image. To produce a visually accurate color image using a digital film recorder, the locations on a frame of film that correspond to each pixel of an image must be exposed precisely. The calculation of this exposure (for each primary component, red, green and blue) depends on the nature of the source and the characteristics of the film used.
Producing a film image that corresponds visually to the original image created and/or stored on a computer is not as straightforward as it might initially appear. Difficulties arise because the color densities produced on a film generally do not correspond linearly to the RGB color values that displayed on the computer monitor. The color densities actually produced on the film are affected by a variety of factors, including chemical characteristics of the film itself. For example, due to chemical characteristics of the film, the density of a particular color produced on the film by a beam of light generally will not vary linearly with the intensity of light used to expose the film for a given time duration. Temperature, film type, characteristics of a light source used in the recorder and noise generated by the system can affect the color density values actually produced.
Similarly, the stored value used to produce a particular measured density for one color component in a neutral tone generally will not produce the same measured density in color. For example, if the combination of red, green and blue code values X1, Y1 and Z1 produce measured red, green and blue densities R1, G1, and B1, then the red code value X1 used in combination with different green and blue code values (i.e., Y2 and Z2) generally will not produce the density R1.
The monitor itself has nonlinear characteristics. The well-known gamma correction process is used to correct for nonlinearities in specific monitors. Nevertheless, even a monitor's characteristics tend to degrade over time. Monitors operate in RGB color spaces in which colors are created by mixing proportions of Red, Green and Blue light. Monitors from different suppliers may use different phosphors and an individual monitor itself will age. This is equivalent to different or gradually changing color spaces. All of these problems and complexities suggest to those skilled in the field the complex color management schemes necessary.
Another problem is that the film cannot reproduce all the colors reproducible on the monitor (the converse is also true, but less important since we're only trying to mimic the monitor). This phenomenon is referred to as “gamut mismatch”, and treating such cases in a consistent manner is difficult. One approach would be to take all the colors that are outside the film's gamut and map them to the closest (in some sense) point on the surface of the film's gamut. Colors which are inside the gamut to start with would be left alone. This approach, however, results in abrupt color changes which result in banding artifacts in the final image.
Industry literature generally teaches that solutions to the above-described problems must be complex. “Historically, managing color has been a very time consuming and costly process in the printing, prepress, and film industries”. Has & Newman, Color Management: Current Practice and The Adoption of a New Standard. “Color is an immensely complex subject, one that draws on concepts and results from physics, physiology, psychology, art, and graphic design. The color of an object depends not only on the object itself, but also on the light source illuminating it, on the color of the surrounding area, and on the human visual system”. See Foley and Van Dam, Computer Graphics Principles and Practice (Second Edition 1996).
In accordance with such theory, conventional approaches to producing color images on film that match the originally created computer graphics images have been complex. For example, some conventional approaches involve the use of complex models of the film development process to change the primary color component values derived from the computer graphics images into values that will produce a similar visual result on film. Other conventional approaches arrive (by trial and error and much manual tweaking) at a transfer curve for each channel such that the colors on the screen look acceptable. Such methods do not allow for fine-grained, color-by-color matching. In any event, it is generally understood that brightness, or intensity, values that are used in connection with the display of images on a computer screen cannot be converted to the film media, as they are irrelevant to the visual perception of the film in the theater environment. In the theater environment, so the reasoning goes, the psychophysical perception of various colors by the audience is affected by the darkness of the theater and numerous other subtle factors that are difficult or impossible to account for in advance. It is generally understood that the intensity value of each color component must be determined by visual inspection in the new medium (i.e., the film projected in a theater environment).
Conventional approaches to dealing with the above matter typically address the reproduction of color on print media, which cannot reproduce colors as well as monitors can. Most importantly, the dynamic range (ratio of maximum to minimum brightness values) is substantially lower than that of monitors. Much attention is therefore focused on the problem of compressing the dynamic range. Film, on the other hand, offers a dynamic range that is comparable (or even larger) than that of monitors, an issue which has not been addressed adequately in the literature.
Efficient and accurate techniques for converting color computer graphics images to film images would be highly desirable.